1. Field of Invention
The present invention relates to a satellite signal reception device for receiving signals from positioning information satellites such as GPS satellites, and to a control method for a satellite signal reception device
2. Description of Related Art
The Global Positioning System (GPS) for determining the position of a GPS receiver uses GPS satellites that circle the Earth on known orbits, and positioning devices that measure the current position of a receiver that receives signals from these satellites are commonly available.
Each GPS satellite has an atomic clock on board. Each GPS satellite therefore keeps the time (referred to below as the GPS time or satellite time information) with extremely high precision.
Time adjustment devices (timepieces) that receive signals (navigation messages) from the GPS satellites, acquire the time information, and adjust the displayed time using the decoded time information have also been proposed.
With a satellite signal reception device (positioning device or time adjustment device, for example) for receiving signals from positioning information satellites such as GPS satellites, the reception level changes according to the location where the signals are received (also referred to herein as the “reception site”). Therefore, in order to determine if the reception conditions are good, the user of the satellite signal reception device is preferably able to know the reception condition expressed as a level (the “reception level” below).
Japanese Unexamined Patent Appl. Pub. JP-A-H10-73650 teaches a car navigation device that displays the reception level of each GPS satellite using a bar graph as an example of a positioning device having a function for displaying the reception level.
While the technology taught in Japanese Unexamined Patent Appl. Pub. JP-A-H10-73650 enables a person that understands the operation of a GPS satellite to determine the reception conditions by displaying the reception level of each GPS satellite separately, there is the problem that it is difficult for the general user that does not know how the GPS system works to determine the reception conditions.
More specifically, if the reception level of all GPS satellites displayed is high, or if the reception level of all GPS satellites displayed is low, the general user will typically decide that reception conditions are good, or that they are poor.
However, when the reception level of some GPS satellites is high and the reception level of other GPS satellites is low, it is difficult for the typical user to determine whether reception may be continued from the current location or whether the user should move to a different site with better reception. More particularly, the positioning process normally requires receiving signals from four GPS satellites. However, if the user does not know this, the user is very likely to continue reception from the same location when the reception level of only one to three GPS satellites is high, and thus continues the reception process even though the position cannot be determined accurately.
A satellite signal reception device enabling even general users to easily and unerringly know the reception condition is therefore desirable.
In order to display the reception level for each of a plurality of GPS satellites as described in Japanese Unexamined Patent Appl. Pub. JP-A-H10-73650, a relatively large display device is needed in order to display the reception levels. While this is possible in a car navigation system such as taught in Japanese Unexamined Patent Appl. Pub. JP-A-H10-73650, it is difficult to use the technology taught in Japanese Unexamined Patent Appl. Pub. JP-A-H10-73650 in a small satellite signal reception device that is typically used worn by the user, such as a wristwatch, because the size of the display device that can be used is small.